In rotary tube kilns, high temperatures are usually used. The rotating tube can, to this end, be heated indirectly to the desired temperature with a heating medium (such as hot gas or hot air), in order to reach, in the interior of the rotating tube, the sufficiently high temperatures (several 100° C., in part >1000° C.) for chemical processes or other desired processes taking place therein. To this end, the rotating tube is usually surrounded by a heating tunnel, as shown, schematically, in FIG. 1, which shows a schematic cross-section through a rotary tubular kiln, according to the state of the art. The heat tunnel 12, surrounding like a housing the rotating tube 10 which turns in the direction of the arrow C (or in the opposite direction), has several burners 14 on its entire length, which indirectly heat the rotating tube, and gas outlets 16. A heating medium, such as hot gas, is introduced through the gas inlets 14A; the medium flows around the circumference of the rotating tube (also called rotating drum) and thus heats it. The gas can flow around both the underside of the rotating tube (as shown by arrow A) as well as the upper side (as shown by arrow B).
The efficiency of the kiln is much greater by providing a flow of heating medium along the upper side of the rotating tube. The kiln efficiency is increased because the residence time of the gas along the surface of the rotating tube is increased and thus more time is provided for heat exchange. Moreover, flow of heating medium along the upper side results in exposure of a larger surface fraction of the rotating tube to the flow of heating medium, i.e. hot gas. Since it is possible for a portion of the gas, however, to flow around the underside of the rotating tube, a loss in efficiency occurs. This results since with such a flow pattern, the heat exchange is clearly less. A loss in efficiency and reduction in heat transfer also occurs if a narrow passage is provided in the form of a slit D along the underside of the rotating tube. In this case, the flow of heating medium would move along arrow A′.
The rotating tube in FIG. 1 is shown circular, schematically. This does not correspond to reality however. Since such a rotating tube extends over several meters, in part up to 100 m, it is technically almost impossible to guarantee a completely round profile over this entire distance. Furthermore, the rotating tube has a certain imbalance.